1. UI created using plain Ext JS 4.x renders directly & properly on various mobile devices
2. If yes,the ways in which the rendering can be made efficient in terms of data transfer(ex. js files) and response time
3. If no, what approach needs to be taken to run the same application over mobile devices

well, ext4 will run on mobile devices, since they use webkit, the same engine that is used in chrome and safari.

however, ext js 4 has no support for touch events and sencha touch has no support for rightclick, mousewheel etc.

so my recommendation is to use ext js 4 for the desktop world and ST 2 for mobile.
you can at least share the models, stores and parts of the controller logic, but when it comes to views, they should be different for obvious reasons.

“There are some other threads regarding mobile dev. Also, you can build a stripped down version of the library using JSBuilder. Also if you're not using any Components at all, you might just be able use Ext Core, rather than the full framework.”

We would like to know that if the above opinion can serve our purpose in any way.